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Breast cancer: what is it?

Figure 1

Normal Cell DivisionA cell grows a bit larger and then divides into two cells

To understand what breast cancer is, it is important to first understand how the body's cells normally work. The body is made up of tiny cells, such as skin cells, muscle cells, heart cells, nerve cells, and bone cells. When a baby grows, the number of cells increases very quickly. A cell becomes a bit larger, then divides into 2 "daughter" cells (see Figure 1). Each of these cells divides again, and so on.

Once a child grows to adulthood, the cell division process still takes place, but more slowly - for example, dead skin flakes off as the skin constantly renews itself. Normally, this renewal process happens in a very ordered, systematic way. Each cell carries genetic "instructions" that control how fast the cell should divide and grow and when the cell should die. A balance between cells growing and dying keeps our bodies functioning normally.

In cancer, cell growth goes out of control

Sometimes a cell starts to grow without regard for the normal balance between cell growth and death, and a small, harmless lump of cells will form. These harmless growths are referred to as benign. A benign growth can occur in any part of the body, including the breast, skin, or intestine.

In other cases a cell may grow and divide with complete disregard for the needs and limitations of the body. These cells have the potential to grow into large masses or to spread to other areas of the body. Cells that have this aggressive behaviour are called malignant. More commonly, a mass of such cells is called a cancer. A cancer that continues to grow can eventually overwhelm and destroy the part of the body or particular organ where it is located - in this case, the breast tissue. When clumps of these cells spread to other parts of the body they are termed metastases.

Cancer cells also have the ability to stimulate the development of blood vessels to increase their own blood supply and enhance their growth. However, sometimes a cancer's wild growth rate backfires. The cancer may grow so rapidly that it can't get enough oxygen and nutrients from the blood vessels. When this happens a part of the cancer may suddenly die. This death of a group of cells within a cancer is known as necrosis.

Figure 2

How Breast cancer may spreadA cancer grows in the breast (A) and spreads by invitation to surrounding tissues of the breast (B) or through the lymph system to lymph nodes (C ) or through the blood stream to distant organs such as the bones, lungs, liver or brain (D)

How breast cancer may spread

Most normal cells stay where they belong and don't spread to other parts of the body. Breast cancer cells, like other cancer cells, disregard this principle and may spread through the body (metastasize) in several ways (see Figure 2). They can directly invade and destroy the organ of origin, or spread through the lymph system or bloodstream to distant organs such as the lungs, liver, and bones.

When a cancer spreads, it retains the properties of the original cancer. In other words, a breast cancer that spreads to the bones is still a breast cancer. Under the microscope it looks different from a cancer that started in the bones, and it responds to treatment like a breast cancer, not a bone cancer. The original cancer in the breast is called the primary cancer. A cancer that has spread to another site is called a secondary or metastatic cancer.

Cancer cells can trick the immune system

The immune system consists of a group of cells called white blood cells, specialized to recognize and destroy "foreign" material in the body such as bacteria, viruses, and unfamiliar or abnormal cells. Cancer cells somehow manage to slip through this detection system without triggering the immune system to start fighting, whether at the primary cancer site, in the blood vessels, or at the site of distant spread.

Figure 3

Breast cancer does not develop overnightGradually the cells become more abnormal-looking or atypical. Eventually, the cells are recognized as being sufficiently abnormal to be called cancer cells. At first they are within the milk ducts (in situ cancer), and later become invasive breast cancer cells

Breast cancer does not develop overnight

It can take years of cells dividing before a normal cell becomes a cancerous cell. The cell first undergoes very small changes in which it becomes slightly abnormal or atypical as seen under a microscope. It may also begin to divide, grow more quickly, and accumulate in excessive numbers (hyperplasia). Then, over the years the cells continue to change, become more abnormal-looking, and finally cancerous (see Figure 3).

Initially the cancer cells are confined within the milk ducts (in situ cancers), but with time the cells develop the ability to invade surrounding tissues and travel into the blood and lymph system. This is called an invasive cancer.

Unfortunately, it is not possible to detect one or a few abnormal or cancer cells. Current technology is only capable of detecting a small lump or mass of cancer cells that has already started to grow. By the time a cancer can be detected as a lump, it may have been growing slowly within the breast for anywhere from 2 to 8 years.

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